<!DOCTYPE html>

<!-- Counts the number of animation frames that have been served to this frame. -->
<script>
let rafCount = 0;
function raf() {
  rafCount++;
  requestAnimationFrame(raf);
};
requestAnimationFrame(raf);

addEventListener('message', (e) => {
  e.source.postMessage(rafCount, e.origin);
});
</script>

